# B How do we know that light that comes from the stars is in past?

#### shipiz

So for some reason, from time to time, i always come back to this question and i can't remember that part of the physics while i was studying and most of the explanation are pretty generic.

Basically how do we know that light actually travels and not just oscillate and transfer energy when continuous flow is established between source and observer ?

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#### russ_watters

Mentor
So for some reason, from time to time, i always come back to this question and i can't remember that part of the physics while i was studying and most of the explanation are pretty generic.

Basically how do we know that light actually travels and not just oscillate and transfer energy when continuous flow is established between source and observer ?
Well, we know that light travels at a finite speed, right?

#### shipiz

Well, we know that light travels at a finite speed, right?
Yeah, and that is fine and completely understandable. But if continuous flow is established between light source and observer, how can we prove that what we actually see is single particle that has traveled from point A to point B, and not just oscillation which transfers energy (something similar to AC current)?

#### russ_watters

Mentor
Yeah, and that is fine and completely understandable. But if continuous flow is established between light source and observer, how can we prove that what we actually see is single particle that has traveled from point A to point B, and not just oscillation which transfers energy (something similar to AC current)?
What you are saying in that last bit really doesn't make any stand-alone sense at all (AC current travels at almost the speed of light too). But taken as a whole, since we have a well established understanding of how light works on earth and in our solar system, there is no reason - no evidence - to believe some totally unrelated process is at work at longer distances.

#### phinds

Gold Member
Basically how do we know that light actually travels and not just oscillate and transfer energy when continuous flow is established between source and observer ?
I don't see how that could possibly account for red-shift that varies with distance, just to name one issue.

#### slow

Basically how do we know that light actually travels and not just oscillate and transfer energy when continuous flow is established between source and observer ?
Experiments with light have been made between the Earth and the Moon. In fact, mirrors have been mounted on the Moon, to send a beam from the Earth, reflect it on the Moon and receive it again on Earth. Those two and a half seconds of round trip have been well proven in those experiments. Any astronomical distance can be considered as a series of stretches, all in the order of the distance between Earth and Moon. If everyone behaves like the space between Earth and Moon, then the time differences estimated by astronomers can not be too wrong. Within the instrumental possibilities, they must be perfectly valid.

#### Dale

Mentor
Basically how do we know that light actually travels and not just oscillate and transfer energy when continuous flow is established between source and observer ?
Because Maxwell’s equations say it does and we have lots of evidence that light obeys Maxwell’s equations.

#### CWatters

Homework Helper
Gold Member

"How do we know that light that comes from the stars is in past?"

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